This chapter is the substantively revised and expanded version of the original contribution to the first edition of the Elgar Encyclopedia of Comparative Law (J. Smits, ed., 2006). It reviews and discusses the theoretical scholarship on the concept of transnational law, going back to Philip Jessup’s introduction of the term in the nineteen-fifties and tracing it to the present day. The chapter highlights the relevance and potential of the idea of transnational in a range of fields, including commercial law (lex mercatoria), corporate law, international human rights law, comparative constitutional law, anthropology, and ‘global administrative law’. The chapter concludes with a number of observations on the growing presence of transnational legal studies in today’s law school curricula, in North America and elsewhere.